Async

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {
print("This is run on the background queue")
dispatch_async(dispatch_get_main_queue(), {
print("This is run on the main queue, after the previous block")
})
})

After:

Async.background {
print("This is run on the background queue")
}.main {
print("This is run on the main queue, after the previous block")
}